1. Field of the Invention
This invention relates to tube cutting machines, and specifically to machines for cutting thin-walled metal tubes without burrs protruding radially from the tube end.
2. Prior Art
It is well-known in the art to have tube cutters, including small hand cutters revolving around a small metal tube as a user tightens a rolling blade into the tube side wall and also including large tube shearing machines, also typically having a rolling blade orbiting around a large stationary tube but including a mandrel within the tube to prevent tube deformation.
Whether the blade rotates around the tube or the blade moves radially into a rotating tube, the cutting action is similar. A rotating blade moves into the tube wall until the blade severs the tube end from the tube body leaving extensive burrs protruding radially inward and outward of the tube wall. This is inherent in the cutting process employed. With the blade moving radially into the tube, tube wall metal is pushed longitudinally with the tube wall to make room for the blade and then necessarily radially outward and inwardxe2x80x94the only place for the metal to go as it can""t move successfully against the remaining tube wall or the blade. The blade continues into the tube wall until it completely penetrates the wall and thus severs the tube end from the tube body, continuing to create displaced metal radial of the tube wall. The displaced metal remains as burrs after the tube end is severed.
This conventional tube-cutting process inevitably requires further processing such as grinding to remove the burrs prior to further application, such as welding or otherwise joining to other assembly members or simply cleaning to avoid inadvertent skin cuts from the burrs. Further processing is generally time consuming and expensive.
It is a primary object of the present invention to provide a method to cut thin-walled tubes without leaving radial burrs, thus obviating the need for further processing to eliminate those burrs.
This primary object is achieved in a method that creates a neck in the tube wall before a cutting blade moving into the tube wall, drawing tube material away from the tube inner wall diameter creating an inner groove as the blade creates an outer groove, the neck forming between the grooves. Necking follows stretching of the tube end from the tube body before the cutting blade, a well-known effect resulting from stretching a material. The cutting blade initially defines a weakened point on the tube wall and then a stretching longitudinal force on the tube further weakens the wall at that point in the necking process.
The longitudinally stretching force can be applied explicitly with an attachment to the tube distal end urging the end away from the tube body as the blade penetrates the tube wall. The force can also be obtained from the blade itself. The blade comprises a body side and an end side joining at a blade edge, each at angles from normal to the tube wall, referred to as body side angle and end side angle. For these purposes, the blade body side refers to the blade side closest to the tube body and the blade end side refers to the blade side closest to the tube cut portion. The portion of the tube being severed away from the larger remaining tube is referred to as the tube cut portion, or distal end; the remaining tube portion is referred to as the tube body, or proximal end. As the blade edge enters the tube wall, the blade end side pushes the tube cut portion longitudinally away from the tube body, causing the necking result. To prevent material from being urged radially outward on the tube body, rather than advancing the blade perpendicular, or normal, to the tube, the blade is advanced into the tube at the body side angle. Thus, there is neither material accumulating on the body side because of the angle of penetration nor on the cut side because the tube is urged longitudinally away from the blade providing room for the blade.
As the blade continues into the tube wall the inner channel becomes greater and the neck becomes increasingly thin until the tube wall fails, not because the blade has penetrated to the tube inner wall but because the neck has thinned and weakened until it no longer can support the tube cut portion, or that portion removed from the tube body. When the tube cut portion thus separates from the tube wall at the broken neck, only the remnants of the neck remainxe2x80x94a minor ridge on the severed ends between their inner and outer walls extending longitudinally.
A tube cut with the new method thus does not have any burrs radially inward or outward of the tube wall, though there is a small burr extending longitudinally from the tube wall.
If the blade continued into the tube after the neck had broken, the blade could bend the remaining neck ridge inward and cause a radially inward burr, defeating the purpose and achievement of the method. It is therefore important to stop the blade from advancing no later than when the neck fails. In practice, the blade is reversed when the neck weakens and is unable support the tube cut portion but has not yet broken, referred to as the failure point. The blade is then withdrawn and the neck is snapped off.
The failure point is detected by a rod urged transversely against the tube cut portion with a rod bias. The rod bias is small such that it cannot move the tube until the failure point is reached. When the rod begins to move the tube cut portion under the rod bias, the blade is immediately withdrawn. The rod continues to move the tube cut portion, snapping it from the tube body. Though the process of removing the blade and snapping the tube cut portion from the tube body is sequential, in effect and appearance it is simultaneous.
In a first embodiment, a tube is held in a chuck with an expanding mandrel. A mandrel shaft positions the chuck in the tube opposite the chuck with a mandrel distal end extending beyond the tube and just slightly beyond the blade. The chuck then rotates with the tube as the blade advances toward the tube, as in a common lathe. The blade and mandrel are mutually positioned so the mandrel extends only slightly beyond the blade edge as it approaches the mandrel through the tube so when the tube snaps off, the tube cut portion easily falls off the mandrel.
Clearly, it doesn""t matter to the severing process whether the chuck rotates the tube and the blade is stationary relative to the tube (except the transverse advance of the blade toward the mandrel) or the chuck and tube are stationary and the blade rotates orbitally around the tube. Therefore, a second embodiment includes a stationary chuck for receiving a tube with the mandrel axially within the chuck and a blade on a tool mount rotating on a ring generally around the chuck and tube. Similarly, if applied to an installed tube, the chuck is not required and the mandrel is secured in the tube relative to the ring. The blade still advances transversely toward the mandrel, in this embodiment from the ring on which the tool mount rotates, controlled by a motor on the tool mount. This approach has the advantage of substantially requiring only the ring with tool to effect the process, which allows the cutting machine to be mobile. Thus, the machine can be taken to the tube, rather than the tube to the machine. This is applicable where the tube is already installed. It is also advantageous where the tube is bent or large or otherwise unsuitable to be rotated in a rotating chuck.
In instances where the tube is bent, a normally extended mandrel cannot move in the tube past tube bends. The compact mandrel is then employed having a reduced longitudinal dimension, sufficient short to move around tube bends.
In the simplest embodiment, the mandrel shaft is inflexible extending a length generally from the chuck to a shaft support sufficiently removed to receive a tube of length up to that distance over the shaft and mandrel. A hydraulic line (conduit) runs from a pneumatic pump and valve on the shaft support through the shaft to the mandrel, actuated by a pneumatic line between a user""s control panel and the valve. In an alternative embodiment, the shaft is flexible and of arbitrary length, typically coiled on a spool and uncoiled as necessary to move the mandrel within the tube to its position opposite the chuck. The tube is then supported at the shaft support independent of the shaft. The hydraulic line continues to run within the shaft to the mandrel. A device to measure the position of the mandrel from the tube at the shaft support assures the mandrel is properly located opposite the chuck and the blade.
The mandrel is of selective size to match the tube being cut. Its outer diameter is slightly less than that of the tube inner diameter and slidable therein, expanding under hydraulic control to grasp the tube from within.